Signal and variability within a Holocene peat bog — Chronological uncertainties of pollen, macrofossil and fungal proxies

Review of Palaeobotany and Palynology 186 (2012) 5–15

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Research paper

Signal and variability within a Holocene peat bog — Chronological uncertainties of pollen, macrofossil and fungal proxies Maarten Blaauw a,⁎, Dmitri Mauquoy b a b

School of Geography, Archaeology and Palaeoecology, Queen's University Belfast, Belfast BT7 1NN, Northern Ireland, UK School of Geosciences, University of Aberdeen, Aberdeen AB24 3UF, Scotland, UK

a r t i c l e

i n f o

Article history: Received 2 January 2012 Received in revised form 18 June 2012 Accepted 20 June 2012 Keywords: radiocarbon chronologies fossil proxy signal and noise raised bog peat deposits

a b s t r a c t A single raised bog from the eastern Netherlands has been repeatedly analysed and 14C dated over the past few decades. Here we assess the within-site variability of fossil proxy data through comparing the regional pollen, macrofossils and non-pollen palynomorphs of four of these profiles. High-resolution chronologies were obtained using 14C dating and Bayesian age-depth modelling. Where chronologies of profiles overlap, proxy curves are compared between the profiles using greyscale graphs that visualise chronological uncertainties. Even at this small spatial scale, there is considerable variability of the fossil proxy curves. Implications regarding signal (climate) and noise (internal dynamics) of the different types of fossil proxies are discussed. Single cores are of limited value for reconstructing centennial-scale climate change, and only by combining multiple cores and proxies can we obtain a reliable understanding of past environmental change and possible forcing factors (e.g., solar variability). © 2012 Elsevier B.V. All rights reserved.

1. Introduction Peat deposits from raised bogs are well-known for storing valuable information about past environmental conditions. Starting in the late nineteenth century with Rutger Sernander, distinct layers within Holocene peat deposits were used to infer separate climatic periods such as the Boreal, Atlantic, Subboreal and Subatlantic (von Post, 1946; Birks, 2008). Peat layers, as well as pollen stored within the peat, were used to build regional to continental-scale climatic histories at millennial time-scales (Dachnowski, 1922; von Post, 1946). Most early palaeoecological studies were done at centennial temporal resolution, sub-sampling peat cores every 10–20 cm or so. Since not every cm was analysed, the records were often discontinuous (see Liu et al., 2012 for implications). However, from the 1970s on Bas van Geel and others at the Hugo de Vries laboratory (University of Amsterdam) aimed to obtain continuous and more highly resolved environmental information, and therefore started working at centimetre, or decadal resolution. They were also among the first to complement pollen analysis with that of non-pollen palynomorphs such as fungal spores and insect remains, both within pollen slides and through macrofossil analysis. Examples of such studies include that of Wietmarscher Moor (van Geel, 1972) and the well-replicated site of Engbertsdijksveen (Fig. 1; Table 1). To enable placing these decadal resolution fossil proxy data in their context (e.g. temporal comparisons with other time series such as that of past solar activity), precise chronologies were needed. Through close collaborations with the radiocarbon laboratory at Groningen University, ⁎ Corresponding author. Tel.: +44 28 9097 3895. E-mail address: [email protected] (M. Blaauw). 0034-6667/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.revpalbo.2012.06.005

an investigation was made to determine how peat cores could be dated at very high temporal resolution. Dense series of bulk peat or macrofossil remains were 14C dated and then matched onto centennial to decadal scale wiggles of the 14C calibration curve (wiggle-match dating; van Geel and Mook, 1989). Later studies aimed to test which peat remains would give the most reliable dates (e.g., Kilian et al., 1995, 2000; Shore et al., 1995; Nilsson et al., 2001; Blaauw et al., 2004a; Brock et al., 2011), or to quantify the chronological resolution of 14C wiggle-match dated peat deposits (e.g., Kilian et al., 2000; Blaauw et al., 2003, 2004a, 2004b, 2007a, 2007b; Blaauw and Christen, 2005, 2011). Periods where the 14C calibration curve (Reimer et al., 2009) shows rapidly declining 14C ages indicate a reduced solar activity. As van Geel and Mook (1989) argued, high-resolution series of 14C dates from peat deposits should show the same multi-decadal scale wiggles as the 14C calibration curve, and thus changes in solar activity could be inferred directly from these wiggles. This prompted van Geel et al. (1996, 1998, 1999) to investigate temporal links between changes in solar activity and climate variability as recorded within the peat deposits. A major drop in 14C ages (in the calibration curve as well as in high-resolution 14 C sequences from peat bogs) starting around 2800 cal BP (calendar years before present, where present is AD 1950) was interpreted by van Geel et al. (e.g. 1996, 1998, 1999) as a decline in solar activity. This decline occurred at the same time as a major transition from highly decomposed peat to much less decomposed peat in northwest European bogs (e.g., Kilian et al., 1995), an event interpreted to have been caused by a shift to a colder and more humid climate in the region. Also archives in other regions recorded a climate change around this time (e.g., van Geel et al., 1996, 1998; Speranza et al., 2002; Chambers et al., 2007). Since the climatic shift occurred at the same time as a

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Fig. 1. Satellite map of Engbertsdijksveen, with approximate coring locations of the four studied sequences. Cores EngI and EngIb were taken from the same location at 10 m distance from each other. White rectangle in inset shows the location of the site within northwest Europe. Produced using Google Maps.

of the surrounding area lies only c. 5–15 m above sea level (asl) and was probably mostly covered by peat for most of the Holocene, except for some low hills reaching up to 20 m asl c. 1–2 km distance from the current northern section of the bog. Further away, hills reach 50–70 m asl at 12 km distance to the east and 20 km to the southwest. The Vecht river reaches its closest distance at c. 8 km northwest from the bog. Over the past decades the site has been studied extensively by palaeoecologists. In the 1970s, van Geel sampled a 235 cm thick vertical section from the deposit, EngI, and analysed this at 1 cm resolution (van Geel, 1978). Radiocarbon dating placed the section at c. 7–1 kcal BP. In the following decades, more research at comparable temporal resolution was carried out on the bog (see Table 1), resulting in cores EngIb (van der Molen and Hoekstra, 1988), EngII (van Geel and Dallmeijer, 1986), EngV (Middeldorp, 1982), EngVII (Dupont and Brenninkmeijer, 1984), EngXIV (Kilian et al., 1995, 2000), EngXV (Blaauw, 2003; Blaauw et al., 2003, 2004a, 2004b, 2004c), and EngXVI (Blaauw, 2003). Here, four Engbertsdijksveen peat cores for which published data are available from pollen, macrofossils and non-pollen palynomorphs were re-assessed. No additional proxies (e.g., humification, stable isotopes, testate amoebae) were available for these cores. Data from EngI was digitized from diagrams in van Geel (1978). Since most fossil proxy data from EngXIV (Kilian et al., 1995, 2000) have not been published as yet, they were not used. The EngXVI profile (Blaauw, 2003)

major decline in solar activity, this provides strong support for the hypothesis of solar forcing of climate change (e.g., Mauquoy et al., 2004). Even so, not all northwest European peat archives recorded this event of climate change (e.g., Blaauw et al., 2004c), and some regions showed a considerable delay (Swindles et al., 2007; Plunkett and Swindles, 2008). This could be explained by autochthonous, internal processes within peat bogs causing some complacent records, e.g. local dry hummock conditions that failed to record rising water tables (core Eng-XV of Blaauw et al., 2004c). Another potential cause for the mismatches is problems with 14C dating and/or age-depth models. Using the latest age-depth modelling techniques (Blaauw and Christen, 2011), here we will produce updated chronologies for four published peat cores, all from within the same raised bog deposit Engbertsdijksveen. Based on these chronologies and their estimated uncertainties, we will compare the timing of several proxies that indicate regional (arboreal pollen) to local conditions (macrofossils, fungal spores) around and within the bog. Finally, we will discuss the variability of proxy signals, and implications for peat-based reconstructions of past climate change. 2. Site and methods Engbertsdijksveen (Fig. 1), eastern Netherlands, is a currently drained Holocene raised bog deposit in a region of low relief. Most

Table 1 Published sequences from the Engbertsdijksveen raised bog peat deposit, ordered chronologically. Periods covered by sequences are approximate and in cal yr BP. Amount of 14C dates. Additional 14C dates for Eng-I were published in Kilian et al. (1995). Name

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7200–1600 2800–1000 7500–4200 6200–4000 3300–1500 3000–2000 4500–2300 7400–4200

19 11 8 6 13 68 57 18

C dates

References van Geel (1978), Kilian et al. (1995) van der Molen and Hoekstra (1988) van Geel and Dallmeijer (1986) Middeldorp (1982) Dupont and Brenninkmeijer (1984) Kilian et al. (1995, 2000) Blaauw (2003), Blaauw et al. (2003, 2004a,b) Blaauw (2003)

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only has macrofossil data, and therefore this profile was left out. Therefore, four cores were analysed; EngI, EngIb, EngVII, and EngXV. Since influx data were unavailable for most of the cores, microfossils will be presented as percentages of arboreal pollen. For each core we also calculated basic Dupont-indices (Dupont, 1986). This index produces composite scores for each core depth based on the relative abundance of specific macrofossils and a simple index reflecting their hydrological preferences. A higher index implies a drier bog surface. Here we assigned an index of 1 (very wet) to Scheuchzeria palustris and Sphagnum cuspidatum, 3 for Sphagnum papillosum, 4 for Sphagnum magellanicum and Sphagnum sect. Acutifolia, 5 for Sphagnum imbricatum, and 8 for Erica tetralix and Calluna vulgaris, not taking into account occurrences of other more minor components. Since E. tetralix and C. vulgaris were recorded as absolute counts of branches rather than percentages, we obtained their approximate percentages through dividing the counts by their maximum occurrences over all cores, and multiplying the ratios by 100%. The fossil proxy data for core EngXV are available in Blaauw (2003), and a selection is presented in Blaauw et al., 2004c. From a vertical wall of a hole dug into the peat bog, samples were taken using three vertically placed 50 × 15 × 10 cm metal boxes (Blaauw et al., 2003). The core was cut into 0.5–1 cm thick horizontal slices. Microfossil slides were prepared using standard methods (Faegri and Iversen, 1989), with 1 tablet of Lycopodium spores added to estimate pollen concentrations (Stockmarr, 1971). Pollen and non-pollen palynomorphs were identified by light microscopy at × 400–1000 magnification, based on reference collections, Moore et al. (1991) for pollen, and van Geel (1978) for non-pollen palynomorphs. At least 500 arboreal pollen grains were counted at each depth. Percentages are expressed against the pollen sum, based on arboreal pollen (AP). Macrofossil remains were pretreated by boiling a 2.5 cc slice of peat for c. 10 min in 5% KOH, followed by rinsing and sieving (100 μm) using demineralised water. The macrofossil remains were identified and counted using a binocular microscope with ×6–50 magnification, with some remains identified at ×400–1000 magnification (epidermal cells) using a light microscope. Macrofossils were identified using a reference collection and the literature (Grosse-Brauckmann, 1972, 1974; Grosse-Brauckmann and Streitz, 1992). Abundances of macrofossils were recorded as number (e.g., seeds) or visually estimated volume percentage (e.g., mosses).

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Age-models for the cores were produced using Bayesian age-depth modelling software Bacon (Blaauw and Christen, 2011). Bacon divides a core into a large number of thin vertical sections, and models the accumulation rate of each section. Accumulation rates are constrained by a prior distribution (a gamma distribution with parameters acc.mean and acc.shape), as is the variability in accumulation rate between neighbouring depths (“memory”, a beta distribution with parameters mem. mean and mem.strength). The “mean” parameters in the above prior distributions describe the mean accumulation rate and memory respectively, whereas respectively the “shape” and “strength” parameters describe how peaked the distributions of accumulation rate and its variability are (higher values indicate higher peaks so more constrained priors). Both the gamma and the beta distributions are always positive (assuring that no negative accumulation rates occur), and values from the beta distribution are always between 0 and 1 (where 0 is no memory between neighbouring depths, and 1 is 100% memory between neighbouring depths). All cores were set to have section thicknesses of 2 cm and prior mean accumulation rates of 20 yr/cm, except for EngI which had section thicknesses of 3 cm and a prior mean accumulation rate of 50 yr/cm (since this core covered longer periods with quite distinct accumulation rates). The shape for the accumulation rate prior was set at 1.2 for all cores, and the prior memory was set at 0.3 (mean) and 4 (strength). The dates were calibrated using calibration curve IntCal09 (Reimer et al., 2009), assuming a Student's-t distribution with wide tails instead of the usual Gaussian distribution (Christen and Perez, 2009). Depths were all set to be increasing with time (i.e. starting at the top of the cores). Therefore, the published depths of EngI and EngVII, which were counted starting from their base, were multiplied by −1. Although Kilian et al. (1995) suggested that bulk peat 14C dates could suffer from age offsets, this could not be confirmed in subsequent research (Blaauw et al., 2004a). Therefore, all 14C dates including bulk peat dates are treated as accurate, without a reservoir effect. All analysis and graphing was done using freely available, open-source software R (R Development Core Team, 2012). 3. Results The regional pollen spectra of core EngXV (Fig. 2) indicate a stable environment with limited human impact and no major changes in

Fig. 2. Regional pollen diagram of core EngXV. Only arboreal pollen (AP) and human impact indicators are shown. Percentages are based on the AP count; tick mark at 10% for unmarked horizontal axes. Grey shades indicate ×20 exaggeration in order to show low percentages. Horizontal bars indicate samples; most are 1 cm thick, while some sections were analysed every 0.5 cm.

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discerned as changed age-depth slopes. Instead of plotting one single “best” age-depth curve, here we plot all likely age-depth models using grey-scales, where darker colours indicate more likely calendar ages. Given the different time-scales that are covered by the four cores, we decided to focus on the period of 4000–1000 cal BP, which is covered by most of the cores at relatively high dating resolution. None of the cores covers the entire period of focus, however most of it is covered by at least two cores. Each of the age-depth models has been obtained independently, without any assumed synchroneity of fossil proxy events. Therefore it is interesting to compare the timings of several proxy events between the cores. Regional pollen such as from trees should reflect their abundances on large spatial scales, and should thus be highly comparable between the cores. However, grey-scale graphs of pollen time series (Fig. 5) show that not all centennial-scale features of the pollen curves are reproduced similarly between the four cores. For example, while Fagus seems to decline around c. 1700 cal BP in EngI, at the same it increases in twin core EngIb. The same happens with Corylus, with some but not all features reproduced between the cores. Also the long-term percentage values of Corylus differ between the cores. Pollen concentration data would have been useful, since fluctuations in single pollen curves plotted as concentration would be independent from those of other pollen species. However, estimates of pollen concentration were not available for cores EngI, EngIb or EngVII. On millennial scales, there appears to be a tendency towards generally hummock conditions between 4 and 3 kcal BP in the four studied cores (Figs 6–8), with on average more moist lawn or hollow conditions later on. However, on centennial to multi-centennial scales, macrofossil records of Sphagnum (Figs. 6, 7), Calluna vulgaris, Erica tetralix (Fig. 7) and fungal spores (Fig. 8) record a low degree of similarity between the cores. This is perhaps not surprising, given the distinct local micro-conditions of the different cores (e.g., hummock, hollow or pool conditions). Sphagnum cuspidatum for example, a good indicator for moist conditions (and thus possibly a cool/moist climate), shows clear increases at a time of declined solar activity in EngI and EngIb, but not that clearly in EngVII or EngXV (in the latter, increases in S. cuspidatum are delayed by some centuries). Therefore in Fig. 9 we also present for all four cores their Dupont-indices (Dupont, 1986), where lower values indicate a higher macrofossil-reconstructed bog surface wetness.

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% ) Sp r c o ha a l ( gn % ) um Se S. ct cu .A sp cu id tif at ol u ia m S. (% (% pa ) ) pi llo su m S. (% im ) br ic at um Sp ( .d % ) iv .o pe rc Sp ul ha a & Sc gnu ca ps he m ul uc sp es o hz er res ia S. (% p p ) al S. al us Er pa ustr tri io lus is s ph tr h re m or is yda ai umpo th ns rellen ode Er (% m (% s io ) ai ) ph n or s (% um ) sp Er in R iop dl hy ho es nc r u ho m sp se o e ra ds R . al R alb ba . a st Sc alb se em i a e S. rpu po ds s C ca s callen yp es e ( er pi sp % ac to ito ) ea su su e ss s C in ee ep y de d id Er per t. s er i a c (% m E. a ce is ) t te et ae (% tra ra p ) lix lix oll le ste en av m e E. es s xc te l .R C tra al l .a lu ix lb C na see a . v vu d (% ul s ) ga lgar ris is la s m rg C .v al e An u l s st te em d lg m A. romaris s s po e se d lifo a ed lia po s l le ifo A. av lia es s O po te xy lif m s O co olia . p cc s al us ee Er us p ds a t ic al ris l lus es ea tr i ro ve s s s tem ot Er le s ts i c Er a (% ic le ) D ales flo ro s w Be se p e o Ty tul ra lle rs i pe a s se n ( nd 10 pe eds %) et. coc. f ni rui Ty d i ts a pe 12 ch l am Ty yd Ty pe os pe 14 po 14 as re m co yc sp el or iu es m

vegetation. Most obvious are fluctuating abundances of trees such as Alnus, Quercus, Corylus and Betula, possibly indicating minor climate variations (see later). Vegetation openness (100%-AP, indicating human impact) fluctuates as well, with a general trend toward more open conditions toward the top of the core. After a broad peak in the middle section of the core (130–100 cm), pollen concentrations become lower in the upper decimetres (90–60 cm). Both coincide approximately with changes in peat accumulation rate (see later). After an initial short phase with Eriophorum (most likely Eriophorum vaginatum), the local vegetation of core EngXV (Fig. 3) has high abundances of Scheuchzeria palustris, indicating a high local water table (zone A). Between 135 and 130 cm depth, Sphagnum papillosum temporarily indicates more lawn-like conditions, but soon after Eriophorum and later Scheuchzeria together with Sphagnum cuspidatum become dominant, the latter two indicating pool conditions (zone B). From 116 cm depth onwards, Sphagnum sect. Acutifolia replaces Scheuchzeria, indicating drier conditions, although S. cuspidatum and S. papillosum occur in fluctuating abundances as well (zones C and D). Above 89 cm depth, S. sect. Acutifolia is dominant, indicating hummock conditions (zone E). Between 74 and 70 cm depth, there is a shift (zone F) toward S. cuspidatum, followed by S. papillosum and finally Sphagnum imbricatum (zone G; Stoneman et al., 1993). This indicates more moist or oceanic conditions (see later). Several of the reconstructed vegetation shifts occur together with peaks in charcoal. Fungal spores of Types 10 and 12 show large fluctuations (Fig. 3), which sometimes, though not consistently, occur together with dry conditions as inferred from the macrofossils. Ascospores and mycelium of Type 14, Meliola ellisii, have been found in contact with sub-fossil Calluna leaves and stems. It has therefore been used as indicating dry mire conditions (hummock microforms) by van Geel (1978), although Yeloff et al. (2007) report conflicting results from peat deposits in the UK and Denmark. In EngXV, Type 14 is generally absent when Calluna vulgaris is not found within the macrofossil record, whereas they tend to occur together in zones C–F. However, Type 14 is consistently present within zone G, while there is no macrofossil evidence for abundant local C. vulgaris in the same zone. Age-depth models for the four studied cores are presented in Fig. 4. The density of 14C dates is higher in cores that were recovered more recently. Within each core, subtle changes in accumulation rate can be

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4. Discussion The pollen, non-pollen palynomorph and macrofossil data from core EngXV add to the growing body of high-resolution and precisely dated fossil proxy data from Engbertsdijksveen and other raised bog deposits. The level of detail and replication provided through such studies gives information regarding Holocene climate and environmental change, but also raises some questions. For example, we can ask how many of the observed changes in fossil proxy records from peatlands can be attributed to climate change (“signal”), and how many to other factors such as internal variability of pool and hummock microforms (“noise”)? It was expected that regional pollen curves would be very similar between the four well-dated peat cores, because they were sampled from within the same raised bog deposit. Indeed, millennial-scale arboreal pollen percentage trends differ little between the four studied cores (Fig. 5). Since core EngVII was sampled at a shallower and thus probably more peripheral location on the bog than the other cores (Fig. 1; Dupont and Brenninkmeijer, 1984, p. 242), this location could have been closer to sources of arboreal pollen. However, the other cores are located closer to some low hills to the north that could have formed sources of arboreal pollen. During the millennial-scale period of interest, Fagus increases and Corylus decreases (Dupont and Brenninkmeijer, 1984). Short-term

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(centennial-scale) fluctuations of Corylus can be attributed to reduced pollen release forced by short-term climate fluctuations such as wet springtime conditions (van Geel, 1978) or late spring frosts (Godwin, 1975, cited in Dupont and Brenninkmeijer, 1984). However, on these shorter time scales, the grey-scale pollen percentage curves are not quite similar between the cores, with differences in timing and shapes of some features, as well as some centennial-scale features which are only recorded within some of the cores. For example, whereas Fagus peaks considerably around c. 2700 cal BP in EngI, this happens several centuries later in EngXV (or, depending on which peak is chosen, several centuries earlier albeit at much lower amplitude), whereas no comparable peaks can be found in EngIb and EngVII at the time. Even though cores EngI and EngIb were sampled within metres of each other (Dupont and Brenninkmeijer, 1984), Corylus fluctuates considerably more in EngIb than in EngI after 2500 cal BP. Multi-centennial trends of Alnus in EngVII seem approximately anti-correlated with those in EngXV, while twin cores EngI and EngIb are similar in not showing such trends. Possible reasons for the above deviations between supposedly synchronous curves include i) errors in 14C dates or age-model assumptions, ii) stable or evolving researcher-dependent biases during pollen counting, iii) different influxes of Corylus, Fagus and/or Alnus to individual sites owing to different distances of sites to forest edges or isolated patches of the trees (e.g., Caseldine, 1984; Janssen, 1984; Bunting, 2003; Sugita, 2004; Broström et al., 2005; Fyfe and Woodbridge, 2012), iv) distinct preservation and/or inwash of certain pollen types in different bog microforms such as hummocks or hollows (no such effect was found for most pollen by Hopkins, 1950 or Clymo and Mackay, 1987), or v) an essentially unknowable combination of interacting factors that would make the behaviour of proxy curves nonlinear and at least partly unpredictable at decadal to multi-centennial time scales (Blaauw et al., 2010). To us it seems unlikely that the deviations could have been caused entirely by errors in the age-depth models. All four sites were dated using at least a dozen 14C dates between 4 and 1 kcal BP, and none of the dates appear to be outlying. All sites were modelled using the same Bayesian approach with similar settings and assumptions (see Site and methods), taking into account all known and measurable uncertainties. Whereas the 14C dated material of EngXV consisted of carefully selected and cleaned above-ground macrofossil remains, the 14C dates of the other three sites consisted of bulk peat. Kilian et al. (1995) inferred a reservoir effect of up to several centuries for bulk peat 14C dates. After correcting for this deviation (estimated at 117 14C yr for EngI and 149 14C yr for EngVII), the resulting chronologies for both sites showed a clear signal for higher local moisture conditions during the period of decreased solar activity around 2800 cal BP. However, Blaauw et al. (2004a) could find no difference in 14C ages between bulk and above-ground macrofossil remains, calling into question the existence of a bulk peat reservoir effect. Thus, pending further research into the existence and/or size of a reservoir effect in bulk peat 14C dates, we did not apply age offsets. If the fluctuations in the regional pollen curves could be assumed to be synchronous, perhaps the 14C-based chronologies could be ‘corrected’ by synchronising the centennial-scale pollen fluctuations across sites. However, this would be problematic since it is not entirely clear which peaks should be aligned between sites (Fig. 5), nor do we know which site has its peaks at the ‘correct’ age (the tuning target). Moreover, tuning of proxy curves introduces the danger of circular reasoning, especially if the same data would later be interpreted in terms of leads and lags between sites (Blaauw, 2012; Swindles et al., 2012). Even more, the proxy signals do not only differ in timing (“horizontal” differences in Figs. 5–8), but also in the shapes of the features themselves (“vertical” differences). No degree of chronological adjustments would enable vertical alignments of the proxy features. Thus, as outlined above, the vertical differences between proxy curves should be attributed to researcher bias (each core was counted by different researchers), different distances

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Fig. 5. Pollen greyscale graphs (Fagus, Corylus and Alnus) for the four analysed Engbertsdijksveen cores. Abundances in %AP (vertical axes). The graphs take into account chronological uncertainties; large light grey areas indicate zones of high chronological uncertainty, whereas narrow dark zones indicate high chronological precision. Proxy uncertainties (i.e. pollen counts) are not taken into account. Bottom panels show reconstructed solar activity (Solanki et al., 2004).

from pollen sources (nearby hills could have caused a spatially variable pollen rain over the bog), taphonomic processes, and/or non-linear behaviour of multiple interacting factors. The fact that macrofossil-based reconstructions differ between the four cores comes less as a surprise to us, since micro-conditions on local bog surfaces have major influences on local vegetation composition. Macrofossils essentially record how vegetation has changed over time within a surface of as little as 5 cm2 (the area of the macrofossil subsamples).

Since fossil fungal spores essentially reflect changes in vegetation (van Geel, 1978; Yeloff et al., 2007), their responses seem even more erratic than those of the macrofossils (Fig. 8). The Dupont index (Dupont, 1986) summarizes bog surface wetness (height of the local water table) based on the moisture preferences of the reconstructed composition of bog vegetation. As such, changes in water table are expected to be reflected by changing Dupont indices in diverse microsites. For example, a change to a cooler and/or wetter climate should cause a rise in the

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Fig. 6. Proxy ghost graphs for the four sites, for Sphagnum imbricatum, S. papillosum and S. cuspidatum, indicating moderately moist to wet local conditions (lawns to hollows or pools). Visually estimated abundances in % (vertical axes). See caption of Fig. 5 for interpretations of grey-scales.

bog's local water table, with corresponding wetter conditions (lower Dupont indices) in hummocks as well as lawns and hollows. However, Fig. 9 shows that even the changes in the Dupont indices are not consistent between the four sites. Long-term features are comparable between cores EngI and EngIb, but at times cores EngI and EngXV appear anti-correlated. Raised bogs should not be seen as mere passive receptors of climate conditions, but instead as relatively complex living ecosystems, with

phenomena such as Moor-atmung (rapid rises and falls of bog surfaces probably caused by interplays between climate, bog hydrology and gas production), internal hydrological dynamics (Swindles et al., 2012), competition between plant species, persistence of species in suboptimal conditions, and even “ecological engineering” in which bog plants such as Sphagnum generate favourable micro-conditions (e.g., Svensson, 1995). Therefore, some individualistic behaviour between local sites (noise) should be expected. The question then becomes to what degree

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Fig. 7. Proxy ghost graphs for the four sites of main macrofossil indicators for local hummock conditions; Calluna vulgaris (counts of large and small branches), Erica tetralix (counts of large and small branches) and S. sect. Acutifolia (visually estimated abundance in %). See caption of Fig. 5 for interpretations of grey-scales. Scales of vertical axes differ between proxies but not between sites.

do peat proxies reflect changes in climate (signal). This is a topic for future research and beyond the scope of the current paper. Core EngI shows very clear evidence for much wetter conditions (Sphagnum cuspidatum increases) during the 2800 cal BP decline in solar activity (see also Kilian et al., 1995; van Geel et al., 1996, 1998). This evidence has been replicated in other regions (Speranza et al., 2002; Chambers et al., 2007) and for other periods when solar activity

declined (Mauquoy et al., 2002, 2004; Blaauw et al., 2004c). However, core EngIb which was sampled within a few metres from the former one, seems to respond with a delay to the 2800 cal BP event (see Swindles et al., 2007 and Plunkett and Swindles, 2008 for evidence of a delay in Ireland), while core EngVII shows a subdued response. Core EngXV, a hummock at the time, shows 1 cm of Sphagnum imbricatum around c. 2750 cal BP, but this disappears soon afterwards. Thus, while

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some cores responded clearly and unambiguously, other cores show a limited response or a delay in timing. Even more, no clear responses are recorded to another inferred period of declined solar activity around 2400 cal BP (Fig. 9). Our results strongly suggest that instead of relying on single cores, for a dependable reconstruction of past climate change we should analyse and combine multiple cores as well as proxies. Alternative, non-climatic causes of proxy changes should always be considered (Blaauw et al., 2010; Swindles et al., 2012). In any case, reconstructions

of past climate based on fossil proxy records from peat bogs should be used with caution. 5. Conclusion Several decades ago, stratigraphical changes within peat deposits were often attributed to internal dynamics (autogenic, cyclical regeneration of peat hummock/hollows as discussed by Barber, 1981).

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Nowadays however, many studies interpret fossil proxy changes in raised bogs mostly as responses to climate change. Most probably the truth lies somewhere between these two extremes. The types of high-resolution palaeoecological reconstructions pioneered by Bas van Geel (e.g., van Geel, 1978) could prove instrumental in quantifying the climate sensitivity of macrofossils, non-pollen palynomorphs and other fossil proxies (for example, testate amoebae) from raised bog peat deposits.

Acknowledgements Bas van Geel is thanked for his dedicated approach to research and for sharing his wide palaeoecological knowledge. Dan Charman and an anonymous referee are thanked for their helpful comments. MB's doctoral research (incl. analysis of core EngXV) was sponsored by the Netherlands Organisation for Scientific Research (NWO-ALW), grant no. 750-19-812. Thanks to two anonymous referees for their valuable feedback.

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